A hidden line of text in a Word document can quietly halve the numbers in your financial report. Then it can smuggle a copy of itself into the clean file you send on. And 144 days after a researcher warned Microsoft, it still works.

Håkon Måløy, a Norwegian data scientist, disclosed the technique on 28 July, having first reported it in March. He withheld the exact payload but described the mechanism in detail. He calls it one of the first public demonstrations of a self-propagating “AI worm” moving through normal workflows in a mainstream office suite. The Register reported the findings this week.

How the worm moves

The trick hides in plain sight. The malicious instructions sit in white, eight-point text. Word strips colour and font size before it hands a document to the model, as The Hacker News noted, so the white-on-white text stays invisible to you but legible to Copilot.

One instruction alters the figures. The other tells Copilot to copy the prompt into the output and hide it, dressed up as a source-tracking rule.

That infected output becomes the carrier. Add the file to your own work, and Copilot does the same thing again. It does not spread on its own. Each hop needs another Copilot drafting or editing step in which the poisoned file enters the model’s context.

The attacker needs no access to your Microsoft account. As Måløy put it, they “only need to share a malicious document with the victim.”

Why Microsoft cannot just patch it

Microsoft confirmed the behaviour in March and shipped two fixes. It blocked the exact wording of the prompt, then upgraded the underlying model to GPT-5.5. Måløy reworded the payload, and it worked again on GPT-5.6 the next day. “Two mitigation attempts, including a model upgrade, did not close the class,” he wrote.

His deeper point is about architecture. A model has to read attacker-controlled content in order to judge whether it is an attack. By then the malicious text is already shaping that judgement.

Relying on the model to catch it, he wrote, is like “asking an interpreter to execute an untrusted program to determine whether that program is safe to execute.” Add a second model to watch the first and you just get “LLMs all the way down.”

Microsoft points to a defence-in-depth strategy that blocks malicious instructions “at multiple points,” and tells customers to treat unknown content with caution and review what Copilot produces.

It made the blunter version of the argument itself in June, writing that “prompting alone is not a reliable security boundary.” Its own answer to AI security has largely been more AI.

The same weakness, everywhere

The Word worm is one instance of a wider problem. Copilot trusts the content it reads. AI assistants that reach into your files keep springing the same kind of leak.

The same week, researchers at Tel Aviv University and Intuit described “hallusquatting,” SecurityWeek reported. AI coding assistants invent software-package names that do not exist, on some tasks 85% to 100% of the time.

Attackers register those names, fill them with malware, and wait for the AI to pull them onto a developer’s machine. It is the flip side of the finding that AI-written code keeps failing security tests.

All of this, as Copilot goes everywhere

The timing is awkward, because Microsoft is pushing Copilot harder than ever. On this week’s earnings call, Satya Nadella confirmed a Copilot “super app” that folds chat, coding and autonomous agents into one product, launching this year.

The company says it has more than 30 million paid Copilot seats. It says Copilot revenue jumped 60% in a quarter, and that weekly use now rivals Outlook and Teams.

Even the regulators circling Copilot are, for now, worried about a different thing: how Microsoft bundled it into Microsoft 365 and raised prices. The security question is quieter and harder.

Måløy’s own advice, short of dropping Copilot, is to treat every document as untrusted and check everything the assistant touches. Which is not far from doing the work yourself.

Get the TNW newsletter

Get the most important tech news in your inbox each week.